The invention relates to pulse-compression techniques, and more particularly to a pulse-compression technique using a fast Fourier transform circuit in a matched filter.
It is known to raise the average transmitted power of a given radar by expanding the pulse length and simultaneously keeping a constant bandwidth so that the range resolution capability of the radar is not reduced. To achieve this, a long pulse containing some sort of a linear frequency modulation or stepwise approximation to a linear frequency modulation is transmitted. Upon reception, the pulse is compressed to permit separation of adjacent range resolution cells.
U.S. Pat. No. 4,237,461 to Ben H. Cantrell on Dec. 2, 1980, discloses a digital pulse-expander-compressor which uses a stepwise approximation to a linear frequency modulation.
This device has poor Doppler tolerance. That is, the autocorrelation function or compressed-pulse waveform that is obtained with the device exhibits large range-time grating lobes in the presence of large Doppler shifts on the echo pulses. These large grating lobes are undesirable as they may give rise to false detections and range, if time discrimination is used to calculate range.
This device also has poor precompression bandwidth tolerance. Before the received pulse is compressed, it is fed to a sample-and-A/D converter. The sample-and-A/D converter produces out-of-band interference and noise foldover as a part of its output. To reject the interference and noise foldover, the receiver is given an approximately rectangular passband. The limited receiver bandwidth results, however, in an unfavorable mismatch to the pulse. This mismatch causes a degradation in the peak-to-sidelobe ratio of the autocorrelation function or compressed-pulse waveform obtained with the device. This degradation is undesirable because it increases the possibility that weak target echos will be hidden by the sidelobes from an adjacent stronger target echo.